Added figure files illustrating why pca sometimes doesn't work so well

Author: mnielsen

fig/chap6/mnist_pca.py

@@ -0,0 +1,32 @@
+"""
+mnist_pca
+~~~~~~~~~
+
+Plot graphs to illustrate the limitations of PCA.
+"""
+
+# Third-party libraries
+from mpl_toolkits.mplot3d import Axes3D
+import matplotlib.pyplot as plt
+import numpy as np
+
+# Plot just the data
+fig = plt.figure()
+ax = fig.gca(projection='3d')
+z = np.linspace(-2, 2, 20)
+theta = np.linspace(-4 * np.pi, 4 * np.pi, 20)
+x = np.sin(theta)+0.03*np.random.randn(20)
+y = np.cos(theta)+0.03*np.random.randn(20)
+ax.plot(x, y, z, 'ro')
+plt.show()
+
+# Plot the data and the helix together
+fig = plt.figure()
+ax = fig.gca(projection='3d')
+z_helix = np.linspace(-2, 2, 100)
+theta_helix = np.linspace(-4 * np.pi, 4 * np.pi, 100)
+x_helix = np.sin(theta_helix)
+y_helix = np.cos(theta_helix)
+ax.plot(x, y, z, 'ro')
+ax.plot(x_helix, y_helix, z_helix, '')
+plt.show()